etc/input.conf: change license to LGPL

This file didn't have a license in the first place. The fact that this
file was supposed to be a template, to be copied and modified by
individual users, might weaken the assumption that it's GPL. (It was
only later that mpv replaced the duplicate code in input.c with
input.conf essentially.)

All involved authors agreed to LGPL. On exception is an anonymous
contribution in commit 116ca0c768. This "veal" could not be found.
So the I key is noted as exception. input.conf does not support
conditional guards, so I'm not sure what to do here. Either we could
argue it's a joint work, or the fact that the I key won't work anyway
with a LGPL mpv might count.

etc/builtin.conf and etc/encoding-profiles.conf are mpv original things,
written by developers who agreed to LGPL. So mark them as LGPL too.

Copyright

@@ -32,7 +32,10 @@ Some libraries are GPLv2+ or GPLv3+ only. Building mpv with Samba support makes
 it GPLv3+.
 
 Source files with specific licenses:
-- everything under etc/ is unknown
+- etc/input.conf is LGPLv2.1+ (possible exception with I key binding)
+- etc/builtin.conf is LGPLv2.1+
+- etc/encoding-profiles.conf is LGPLv2.1+
+- everything else under etc/ is unknown
 - everything under DOCS/man/ is GPLv2+
 - sub/osd_font.otf is LGPLv2.1+
 - some tests under waftools/fragments/ might be GPLv2+
@@ -353,6 +356,4 @@ x   video/out/win_state.*           unknown
 Other:
 x   Build system                    nominally LGPL, but might contain GPL traces
 x   English translations            removed, but traces might be in C code
-x   etc/input.conf                  unknown
 x   Logo                            GPL (?), might have to exclude from LGPL builds
-    version.sh                      LGPL